Robin McKie
It is the Holy Grail of physics, and it has eluded scientists for the past 30 years. Now researchers believe they are on the threshold of detecting the most elusive force in the cosmos: gravity waves. But if they find the waves do not exist, that would still be significant: it would prove Albert Einstein wrong.
Gravity waves are ripples in the very fabric of the universe triggered by catastrophic events – supernovae, black holes and neutron stars – on the other side of the heavens. They are the key to understanding the nature of the universe, and could even be used one day as a means of interstellar communications, say scientists.
To date, no one has spotted a gravity wave, but several competing groups are finalising giant detectors which they believe will soon pick up these ghostly cosmic emissions.
“We should start running experiments in a few months, although we may not detect gravity waves for another couple of years,” said Professor Jim Hough of Glasgow University’s physics department.
The Glasgow team’s detector is known as Geo-600 and has been built in collaboration with researchers at Cardiff and in Germany. The 4-million device has two giant arms, each 600m long.
When gravity waves strike the machine, they should stretch and shrink these arms in slightly different ways. Detectors at the end of each arm should reveal this discrepancy.
“The trouble is that gravity waves have such a weak effect,” said Hough. “They will distort these arms, but virtually imperceptibly, so we have had to design instruments that can detect a movement of less than a million-million- millionth of a metre.”
This feat of measuring will be carried out using laser beams which will be shone down each arm on to silicon mirrors. The beams will then be flashed back to a central detector. An incoming gravity wave will distort the beams in slightly different ways, and reveal its existence to eager scientists.
Certainly, the discovery of gravity waves will cause considerable relief. They are a key prediction of Einstein’s theory of general relativity, but have remained stubbornly elusive.
They have built increasingly sensitive instruments, which have responded to all sorts of effects: a workman dropping a spanner, children playing football. However, Geo-600 should be able to detect subtle differences between earthly vibrations and harbingers of stellar catastrophes.
Rival machines in California, Japan and Italy are also scheduled to start up next year. “It’s a bit of a race, but we are all collaborating and helping each other,” said Hough. “We will never be confident we have detected a gravity wave and not some local seismic disruption unless we have confirmation from detectors in other continents.”
The new machines will do more than just detect these ripples – they will be able to analyse the properties of gravity waves. This will provide invaluable insights into the objects that emitted them.
Gravity waves are given off when stars throw their weight around, for instance when they collapse into black holes. Other types of waves are given off by pulsars, super-dense stars that rotate as fast as spinning tops.
“We are creating a completely new sort of observatory, one that will allow us to peer into the hearts of stars, particularly those undergoing catastrophic deaths,” said Hough. “Ultimately, we may be able to use our knowledge to send messages across space.
“However, we will be happy just to detect waves to begin with. We should do so by 2001. If we don’t, we will keep going until the end of the next decade. If we get nothing then, we will have to tear up our ideas about cosmological theory and Einstein.”